The present disclosure relates to an image forming apparatus with a plurality of optical scanning devices and particularly to a technology for suppressing a scan position shift which occurs due to a temperature difference between optical scanning devices.
Conventionally, an image forming apparatus has been known which includes an image forming unit configured to form a toner image on a surface of a photoconductive drum and provided for each of a plurality of colors. The respective image forming units are arranged along a conveying direction of a recording sheet above a conveyor belt for conveying the recording sheet and transfer toner images of the respective colors to the recording sheet conveyed in the conveying direction in a superimposing manner.
In the image forming apparatus of this type, each image forming unit deflects laser light output from a light source by a scanning lens made using optical resin with good optical properties after reflecting the laser light by a rotational polygon mirror which is driven and rotated, whereby the laser light is scanned across the surface of the photoconductive drum at a constant speed. In this way, an electrostatic latent image is formed on the photoconductive drum surface.
Here, toner images of the respective colors formed on the photoconductive drums by attaching toners to electrostatic latent images need to be transferred in a superimposing manner so as not to cause any position shift on a recording sheet. To this end, a control is executed to adjust write positions (scan positions) of the electrostatic latent images on the surfaces of the photoconductive drums. For example, a control is executed to adjust the operations of the other rotational polygon mirrors so that the rotational polygon mirror in a certain image forming unit and those in the other image forming units rotate with predetermined phase differences.
However, if the temperatures of the respective image forming units differ due to usage frequencies and arranged positions of the respective image forming units, refractive indices of the optical resins forming the scanning lenses may change to be different from each other according to temperature. This may shift laser light paths among the respective image forming units. Even in the case of executing the above control, laser light scan positions may shift among the respective image forming units.
One conventional technology is known which suppresses laser light scan position shifts occurring due to such temperature differences among respective image forming units. According to this conventional technology, when image recording is performed by operating one of a plurality of image forming units, heating means of optical scanning devices in the other image forming units are also operated so that temperature differences of the optical scanning devices in the respective image forming units fall within a predetermined range.
Further, according to another conventional technology, an image of a specific color is formed by rotating a rotational polygon mirror necessary to form the image of the specific color at a rated rotating speed and other rotational polygon mirrors at a rotating speed lower than the rated rotating speed. This enables reductions in noise, vibration, smear of the optical scanning devices and the like while suppressing scan position shifts at the time of image formation.
However, in the case of rotating the rotational polygon mirrors of the other unused optical scanning devices at the rotating speed slower than a predetermined rotating speed suitable for an image forming operation during a single-color image forming operation using only one optical scanning device, the temperature of the one optical scanning device is less likely to decrease since only the rotational polygon mirror of the one optical scanning device is driven and the temperatures of the other unused optical scanning devices increase at a rate slower than the one optical scanning device. As a result, it may take time to sufficiently reduce temperature differences among the respective optical scanning devices.
Further, if the rotational polygon mirror of the one optical scanning device is stopped after the image forming operation is finished, the temperature of the one optical scanning device is likely to decrease. However, when an image forming operation is performed next using that one optical scanning device, it may take time to accelerate the rotating speed of the rotational polygon mirror to a predetermined rotating speed suitable for the image forming operation from a stopped state.
An object of the present disclosure is to suppress scan position shifts which occur due to temperature differences among respective optical scanning devices and enable a next image forming operation to be quickly performed.